Langasite wafer and method of producing same

ABSTRACT

The invention relates to the field of electronics and can be used in acoustic electronic frequency-selective devices in surface acoustic waves (SAW) and volumetric acoustic waves. The purpose of the invention is the formulation of an industrial process to develop stoichiometrically structured monocrystals of lanthalum gallium silicate, of no less than 75 mm in diameter and greater than 3,5 kg in weight, along a direction of &lt;01.1&gt;±3°. The discs are cut out at a 90° angle with respect to the lengthwise axis, thereby ensuring that the value of the frequency temperature coefficient is zero.

TECHNICAL FIELD

The present invention relates to radioelectronics and can findapplication in acoustoelectronic frequency-selective devices operatingon surface acoustic waves (SAW) and volume acoustic waves (VAW).

BACKGROUND ART

SAW- and OAW-devices possess great potentialities whose practicalrealization involves developing new materials and cheaper techniques ofgrowing piezoelectric crystals having preset quality and dimensions.

Single crystals of langasite (La₃Ga₅SiO₁₄) prove to be a promisingpiezoelectric material useful for providing modern radioelectronicequipment with small-size selecting devices as possessing a set ofdefinite properties, i.e., langasite crystals are superior to berliniteas to dielectric constant and to quartz as to Q-factor. Their triclinicsymmetry allows for the existence of cuts having a low or even zerotemperature coefficient of frequency with an adequate value of theelectromechanical-coupling coefficient.

Implementation of the new material, i.e., langasite will allow ofreducing the cost of crystals, developing technology of their industrialproduction, and increasing the dimensions of grown crystals.

The closest to the proposed wafer and method of its production as to thetechnical essence and attainable result is a disk-shaped langasite waferfor SAW-devices (cf. K. Shimamura et al. “Growth and characterization oflanthanum gallium silicate La₃Ga₅SiO₁₄ single crystals for piezoelectricapplications”. J. of Crystal Growth, 1996, v.163, pp.388-392). Thediameter of langasite wafers is not in excess of 50 mm because the knowntechniques of growing langasite crystals fail to produce qualitycrystals of a larger diameter. However, production process equipmentused by the majority of domestic and foreign firms is designed for aminimum disk diameter of 76 mm (3 in), since said size minimizes lossesin the devices along the disk periphery with respect to a-total numberof devices on the entire surface of the disks.

One prior-art technique of growing single crystals of langasite by theCzochralski method is known to comprise high-frequency induction meltingof a charge in a platinum crucible, said charge having been synthesizedfrom a mixture of oxides of lanthanum, gallium, and silicon using themethod of solid-phase synthesis, followed by air-pulling of the crystalfrom the melt onto an oriented seed (M. F. Dubovik et al., “Langasite(La₃Ga₅SiO₁₄), an optical piezoelectric growth and properties”, 1994,IEEE International frequency control symposium, 1994, pp.43-47). Themethod allows of growing langasite crystals having a diameter of 60 to70 mm and a weight of 1 kg, using a cylinder-shaped crucible 100 mm indiameter which is nearly equal to its height. The thus-grown crystalsare then annealed at a temperature of 1623 K.

One more prior-art technique of growing single crystals by theCzochralski method is known to comprise high-frequency induction meltingof presynthesized charge in a platinum crucible and pulling the crystalsfrom the melt onto an oriented seed in the atmosphere of nitrogen dopedwith O₂ (3 vol. %) (A. A. Kaminski et al. “Investigation of trigonal(La_(1-x)Na_(x))₃Ga₃SiO₁₄ crystals”. Phys. stat. Sol. (a), 1983, v.80,pp.387-398). However, crystal growing in the atmosphere of pure nitrogenis accompanied by substantial evaporation of gallium oxide Ga₂O₃, whileadding oxygen increases platinum content of the melt.

The inventors of another known technique of growing langasite crystalswere to solve the problem of developing industrial-scale technique ofgrowing langasite crystals by improving the construction arrangement ofthe thermal unit of the crystallization chamber (A. N. Gotalskaya et al.“Aspects of growing langasite crystals and their properties”. Journal dephysique IV, 1994, v.4, pp.201-210). The result was the grown crystals62 mm in diameter and up to 2 kg in weight. Crystals are grown from acharge prepared by the method of solid-phase synthesis.

The closest to the proposed method as to the technical essence and theattainable result is a method involving the Czochralski technique ofgrowing a langasite crystal, comprising charging a crucible with apresynthesized material corresponding to La₃Ga₅SiO₁₄ as for itscomposition, creating a shielding atmosphere followed by melting thefeed material, bringing the rotating oriented seed crystal in contactwith the melt surface, and pulling the oriented crystal from the melt(cf. K. Shimamura et al. “Growth and characterization of lanthanumgallium silicate La₃Ga₅SiO₁₄ single crystals for piezoelectricapplications”. Journal of Crystal Growth, 1996, v.163, pp.388-392).According to the known method, crystals are grown in an induction-heatedgrowing apparatus. Once the feed material has been charged in a platinumor iridium crucible, crystals are grown in the stream of a gaseousmixture consisting of argon and oxygen (1-2 vol. %).

However, crystals grown by the known technique exhibit the onset ofsecond phases, the properties of crystals are unreproducible fromprocess to process, and numerous optically visible scattering centersappear. The situation is aggravated when attempt is made to growcrystals having a diameter exceeding 70 mm. Moreover, use of crystalsgrown by the known technique involves a heavy loss in material duringmanufacture of wafers because the latter are cut off at a large angle tothe crystal growth axis.

The closest to the proposed method of preparing a synthesized material(charge) for growing single crystals of langasite is a method ofpreparing the charge by solid-phase synthesis by sintering the startingoxides of lanthanum, gallium, and silicon (B. V. Mil et al. “Modifiedrare-earth gallates having the structure of Ca₃Ga₂GeO₁₄”. Transactionsof the USSR Academy of Sciences, 1982, v.264, # 6, pp.1385-1389).

According to the known method, the oxides are mixed in a stoichiometricratio, then sintered in an oxygen-containing atmosphere at 1300° C.However, the thus-prepared charge cannot be used for growing singlecrystals of langasite having a stoichiometric-ratio composition, sincesintering of the metal oxides is accompanied with loss of the volatilecomponent thereof. Lowering the sintering temperature below 1300° C.allows of reducing the loss of the volatile component which, however,results in a lower yield of lanthanum gallosilicate due to an incompleteproceeding of the reaction as to volume. To render the synthesis processmore complete necessitates repeating the comminution of the synthesisproduct and mixing the oxides followed by their heating. This in turnresults in fouling the end product, i.e., the langasite charge and hencein higher manufacturing cost thereof.

DISCLOSURE OF THE INVENTION

The present invention has for its principal object to provide anecologically pure industrial-scale method of growingstoichiometric-composition single crystals of langasite having a minimumdiameter of 76 mm (3 in) (of an inscribed circle) and a weight exceeding3.5 kg, since said dimension minimizes the loss in devices along thedisk periphery with respect to the entire surface of the disks. Theresultant crystals should be free from optically detectable scatteringcenters and have such an orientation that wafers produced therefrom beso oriented that the value of the temperature coefficient of frequencyapproximates zero and loss in material be minimized.

The foregoing object is accomplished due to the fact that a disk-shapedlangasite wafer for use in devices operating on SAW has a diameter of 75to 80 mm and the disk has a base cut.

Furthermore, the foregoing object is accomplished due to the fact thatin a known method of growing single crystals of langasite, using theCzochralski technique, comprising loading a crucible with apresynthesized material (charge) corresponding to La₃Ga₅SiO₁₄ as for itscomposition, creating a shielding atmosphere followed by melting thefeed material, bringing the rotating oriented seed crystal in contactwith the melt surface, and pulling the oriented crystal from the melt,said bringing the oriented seed crystal in contact with the melt surfaceis preceded by holding the molten material for 2 to 15 hours, theshielding atmosphere is established using a mixture of argon or nitrogenwith atmospheric air taken in amount of from 2 to 15 vol. % with a totalpressure of from 1.10 to 1.80 atm, which pressure is reduced, prior tocontacting the seed crystal with the melt, to a value falling within therange of 1.00 to 1.09 atm.

The oriented seed crystal is rotated at 20 to 35 rpm.

Further on, used as the oriented seed crystal is a langasite crystaloriented in the direction of <01.1>±3°.

According to the present invention, once the crystals have been grown,they are held for 20 to 36 hours in the argon atmosphere at atemperature of from 1300 to 1673 K and a pressure of from 1.1 to 1.8atm.

The foregoing object is accomplished also due to the fact that in amethod of preparing a charge for growing single crystals of langasite,comprising mixing the oxides of lanthanum, gallium, and silicon,followed by heating the resulting mixture, metallic gallium is added tothe mixture of oxides in the following ratio:La₂O₃:Ga₂O₃:SiO₂=1:(0.2-0.3):(0.68-0.55):0.12, and heating is conductedin the presence of an oxidant locally and transiently before thereaction of spontaneous high-temperature synthesis starts running.

Before being heated the gallium-doped mixture of the oxides isbriquetted.

The local transient heating is carried out in an electric arc.

The essence of the method consists in that holding the melt within anexperimentally found time interval ill combination with a change in thepressure of the shielding atmosphere in the specified range allows of acomplete melt homogenizing process with a minimized loss of the volatilecomponent (i.e., gallium suboxide).

The inventors have established experimentally an optimum range ofchanges in the value of the pressure of the shielding atmosphere, themelt holding time, the rotational speed of the seed crystal and itsorientation, heat-treatment conditions, all this enabling, inconjunction with other essential features of the method, to attain theforegoing object of the invention. With the langasite seed crystaloriented in the direction <01.1> the crystals grow in said direction,whereby wafers can be cut off from the grown crystals at the rightangles to the growth axis, thus minimizing loss of the material and azero-approximating temperature coefficient of frequency. Thus, theproposed orientation of the langasite seed crystal allows of growingalong the direction <01.1>.

The heat-treatment of the grown crystals carried out under the proposedconditions makes possible producing langasite crystals free from anymechanical stresses and optically detectable scattering centers. It isalso worth noting that the proposed heat-treatment of single crystals oflangasite modifies coloration of the crystal specimens. The opticaltransmission spectra of crystals are illustrated in FIG. 1. The testspecimens are 2-mm thick langasite wafers before and afterheat-treatment (cf. characteristic curves 1 and 2, respectively).

In order to prepare a charge for growing stoichiometric compositionsingle crystals of langasite, metallic gallium is added to the mixtureof oxides of lanthanum, gallium, and silicon in the range ofconcentrations as proposed herein. Then the mixture is heated locallyuntil spontaneous high-temperature synthesis starts proceeding in, e.g.,an electric arc. With such a heating, gallium undergoes oxidationaccompanied with heat evolution, which heat renders the process, withthe preselected concentrations of metal oxides and of metallic gallium,into the spontaneous high-temperature synthesis. Gallium oxide resultantfrom the combustion process reacts with lanthanum oxide to form anintermediate phase which reacts, on reaching a temperature of about 1673K resulting from afterburning, with gallium and silicon oxide untilLa₃Ga₅SiO₁₄ is formed. The specific values of the selectableconcentrations of gallium oxides are tabulated below.

Charge composition, g La₂O₃ SiO₂ Ga₂O₃ Ga_(met) 48,04 5,77 28,83 12,4948,04 5,77 32,73  9,59 48,04 5,77 31,99 10,14 48,04 5,77 31,36 10,6148,04 5,77 30,16 11,50 48,04 5,77 28,83 12,49 48,04 5,77 26,44 14,2748,04 5,77 24,05 16,05 Composition of resultant product, % La₃Ga₅SiO₁₄LaGaO₃ b Ga₂O₃ Ga Base 2,5 None None Base 3,5 0,5 None Base 3,0 NoneNone Base 3,5 None None Base 3,0 None None Base 2,5 None None Base 2,5None None Base Melt 10 + 3,5 (SiO₂ *3%)

The resultant charge material features a stoichiometric composition andis suitable for growing single crystals of langasite having a presetquality.

BRIEF DESCRIPTION OF THE DRAWINGS

In what follows the present invention will now be disclosed in adetailed description of an. illustrative embodiment thereof withreference to the accompanying drawings, wherein:

FIG. 1 illustrates optical transmission spectra of crystals; and

FIG. 2 illustrates construction arrangement of a langasite wafer havinga diameter D, a thickness H, and a base cut with a length 1.

BEST METHOD OF CARRYING OUT THE INVENTION

To produce langasite wafers having a minimum diameter of 75 mm, firstcrystals are to be grown, after having preliminarily prepared a startingmaterial (charge), using spontaneous high-temperature synthesis. Toprepare 100 g of the charge, there are taken grade III° C.−• lanthanumoxide (according to Branch Standard C¶b 48-194-8, grade 4 gallium oxide(according to Standard Specifications b‰ 6-09-5729-80), silicon oxide(Standard Specifications b‰ 48-4-360-75, and grade 99.99 metallicgallium (according to Standard Specifications b‰ 48-4-350-75. There aremixed together 48.04 g of 99.99% purity lanthanum oxide, 28.83 g of99.99% purity gallium oxide, 5.86 g of 99.99% air-purity silicon oxide,and 12.49 g of molten metallic gallium. To attain more complete runningof the reaction, the resultant mixture is briquetted into tablets whichare placed in a horizontal reaction vessel to which oxygen is fed at arate of 5 l/h. Then the mixture is subjected to local heating until thereaction of spontaneous high-temperature synthesis starts running in anelectric arc. The synthesis time is from 5 to 15 min, the oxygen feedrate in afterburning is 5 l/h, the cool-down time to room temperature is40 to 60 min. The starting charge prepared by the proposed methodcorresponds to the composition of La₃Ga₅SiO₁₄. The charge taken in anamount of 6.5 kg is loaded in a crucible made of 99.99% purity iridiumand having a diameter of 120 mm made of 99.99% purity iridium. Then thecrucible filled with the charge is placed in the chamber of a crystalgrowing apparatus, the chamber is evacuated to a pressure of 10⁻⁴ Hg,and an argon-air mixture is admitted thereto till a pressure of 1.2 atmis attained. The air is preliminarily dried with liquid nitrogen in anitrogen trap. The air concentration in the argon-air mixture is 10 vol.%, the purity of argon is 99.998%. The crucible is induction-heated tilla complete melting of the charge. A total content of admixtures in themelt as determined by the mass-spectrometric analysis is not in excessof 5×10⁻⁴ wt. %. The resultant melt is held for 15 hours prior tobringing the melt surface in contact with a seed crystal oriented alongthe direction of <01.>±15, and the pressure of the argon-air mixture inthe growing chamber is reduced to 1.05 atm. Then the rotational speed ofthe seed crystal is set at 28 rpm, the seed crystal is brought incontact with the melt surface, and the oriented langasite crystal ispulled from the melt at a rate which changes in the course of growingfrom 2.5 to 1.5 mm per hour. The resultant crystal has a weight of 3.65kg and a diameter of an inscribed circle of the cylindrical portionequal to 80 mm. Once the crystal has been grown completely, it is cooleddown in the growing chamber to room temperature for at least 24 hours.Next the volumetric crystal is held at 1423 K for 24 hours in an argonatmosphere at a pressure of 1.6 atm. Checking the crystal in the beam ofa helium-neon laser for presence of scattering centers thereindemonstrates the crystal to be free from such centers. Moreover, theheat-treatment procedure mentioned before avoids any mechanical stressesin the resultant langasite crystal and modifies coloration of thespecimen crystals. The thus-grown crystals have the orientation of<01.1>. Wafers are cut from the crystals at the right angles to thegrowth axis.

Industrial Applicability

The langasite wafer shaped as a dia. 75-80 mm disk, proposed in thepresent invention can find widespread application in SAW-devices. Theproposed methods of growing langasite crystals and preparing a chargefor crystal growing allows of producing stoichiometric-compositionsingle crystals having a minimum diameter of 75 mm and a minimum weightof 3.5 kg. The process of growing langasite crystals is ecologicallypure and the resultant crystals are free from optically detectablescattering centers.

What is claimed is:
 1. A method of preparing a charge for growing singlecrystals of langasite, comprising mixing the oxides of lanthanum,gallium, and silicon, followed by heating the resultant mixture,CHARACTERIZED in that metallic gallium is added to the mixture of oxidesin the following ratio: La₂O₃:Ga₂O₃:SiO₂=1:(0.2-0.3):(0.68-0.55):0.12,heating being conducted in the presence of an oxidant locally andtransiently before the reaction of spontaneous high-temperaturesynthesis starts running.
 2. The method of claim 1, CHARACTERIZED inthat a local transient heating is carried out in an electric arc.
 3. Amethod of growing single crystals of langasite by the Czochralskimethod, comprising loading a crucible with a charge corresponding to thecomposition of La₃Ga₅SiO₁₄, creating a shielding atmosphere followed bymelting the charge, bringing the rotating oriented seed crystal incontact with the melt surface, and pulling the oriented crystal from themelt, CHARACTERIZED in that loaded in the crucible is the chargeprepared as claimed in claim 1, the molten charge is held for 2 to 15hours, the shielding atmosphere is established using a mixture of argonor nitrogen with atmospheric air taken in amount of from 2 to 15 vol. %with a total pressure of from 1.10 to 1.80 atm, which is reduced a valuefalling within the range of 1.00 to 1.09 atm prior to contacting theseed crystal with the melt.
 4. The method of claim 3, CHARACTERIZED inthat the oriented seed crystal is rotated at 20 to 35 rpm.
 5. The methodof claim 3, CHARACTERIZED in that used as the oriented seed crystal is alangasite crystal oriented in the direction of <01.1>±3°.
 6. The methodof claim 3, CHARACTERIZED in that once the crystals have been grown,they are held for 20 to 36 hours in the argon atmosphere at atemperature of from 1300 to 1673 K and a pressure of from 1.1 to 1.8atm.
 7. A langasite disk-shaped wafer for use in devices operating onsurface-acoustic waves, CHARACTERIZED in that it is cut from singlecrystals of langasite as prepared according to claim 3, the diskdiameter is in the range of from 75 to 80 mm, and the disk has base cut.8. The method of claim 1, CHARACTERIZED in that before being heated themixture of the oxides of lanthanum, gallium, and silicon is briquetted.